This document relates to detection of vascular abnormalities, for example, detection of brain aneurysms.
Computer-aided systems have been proposed for locating possible aneurysm areas from an image of a vasculature volume, for example, to assist diagnosticians with their interpretations and to reduce diagnostic times. In general, prior detection schemes are dependent on the imaging modality, for example, because both the segmentation of the vessels and the false positive reduction use image characteristics.
The detection of brain aneurysms plays a key role in reducing the incidence of intracranial subarachnoid hemorrhage (SAH) which carries a high rate of morbidity and mortality. The majority of non-traumatic SAH cases are caused by ruptured intracranial aneurysms and accurate detection can decrease a significant proportion of misdiagnosed cases. Subarachnoid hemorrhage (SAH) is a serious cause of stroke which affects 30,000 patients in North America annually. SAH accounts for a quarter of cerebrovascular deaths, with 80% of the non-traumatic SAH cases being caused by a ruptured intracranial aneurysm. An intracranial aneurysm is a localized pathological dilatation of a blood vessel. It is reported that up to 2% of the general population harbors aneurysms. Most of these aneurysms are asymptomatic and remain undetected with only a small proportion proceeding to rupture and consequent SAH, with an annual incidence of approximately 1%. However, in the case of a ruptured aneurysm, the initial bleed is fatal in 10-20% of instances and despite improvements in patient management, the incidence of SAH has not declined over time and the morbidity rate is still reported between 25% and 50% in patients surviving aneurysm ruptures.
Detecting intracranial aneurysms from imaging scans is an important step in the prevention of aneurysmal SAH and its attendant complications, as treatment of aneurysms using endovascular or surgical methods carries a lower rate of complication when performed in unruptured versus ruptured aneurysms. Although aneurysm detection is currently performed visually by experienced diagnosticians, there is an increasing interest in computed-aided diagnostic (CAD) systems to assist diagnosticians and possibly improve diagnostic accuracy, while limiting missed detection.
Recent advancements in imaging technologies and the increasing use of less invasive computed tomography (CT) and magnetic resonance imaging (MRI) in outpatient settings, has led to an increased detection of incidental, asymptomatic unruptured intracranial aneurysms (UIA) during the routine evaluation of headache, dizziness, and trauma. While, with rare exceptions, the recommendation is for all unruptured symptomatic aneurysms to be treated, the management of asymptomatic UIA remains controversial. Recent studies estimate the annual rupture rate at only 0.1-0.2%, in contrast with earlier data which reported an annual rupture rate of 1-2%. Since preventive treatment carries risks of complications which increase with age, the decision to intervene and treat UIA needs to be balanced against the risk of rupture. To this end, the International Study of Unruptured Intracranial Aneurysms (ISUIA) released the conclusions of two studies in 1998 and 2003 with the goal of defining an optimal treatment management of UIA. The size and location of the aneurysms were concluded to play important roles in predicting rupture risk and a threshold of 7 mm diameter was proposed.
Under the influence of ISUIA studies, the largest diameter of the aneurysm is a commonly used index to predict rupture. However, it is well known that many small aneurysms rupture, whereas other large aneurysms never do. The aspect ratio (aneurysm height/neck width) is another popular size index used to predict the risk of rupture, especially for small aneurysms which might be missed by the largest diameter measure. A threshold value of 1.6 was recommended for the aspect ratio (AR) to discriminate between unruptured and ruptured aneurysms. While proving useful in many cases, there are studies which report finding no statistically significant difference in AR between ruptured and unruptured aneurysms or dispute how to best use the measure.
Cerebral aneurysms have various shapes and sizes and like size, shape is likely to have an impact on the rupture risk. With advances in medical imaging, modalities such as 3D rotational angiography (3D-RA), computed tomography angiography (CTA) and magnetic resonance angiography (MRA) can capture the complexity of the volumetric shape and offer the possibility to analyze aneurysms in a 3D environment. Still, the morphological characterization of brain aneurysms is an open research area.